The present invention relates to a clamping device for clamping a workpiece to a chuck of a lathe, a grinding machine (for cylindrical or internal grinding), a machining center or the like, and more particularly relates to a drawing-in clamp device of the kind mentioned which can considerably improve a degree of machining accuracy.
A conventional clamping device for clamping a workpiece to a chuck of a lathe or the like as shown in FIGS. 1 and 2 comprises a plurality of (hard or soft) clamp jaws 2 symmetrically disposed about the center line 1 of a scroll chuck and equiangularly spaced apart from each other. Master jaws 5 support the respective jaws 2 and are adapted to slide in T-section channels 4 in a chuck body 3. In clamping a workpiece, racks 6 on the rear surfaces of the master jaws 5 are driven by a spiral scroll 7 incorporated in the chuck body 3 to radially move all the jaws 2 in unison as indicated by double-pointed arrows 8. In FIG. 1, reference numeral 9 represents a bevel gear for acutating the scroll 7 to rotate about the chuck center line 1; and 10, a recess into which a handle rod (not shown) is pushed.
In a conventional hydrualic or pneumatic chuck as shown in FIG. 3, wedge-like members 12 are disposed around the chuck center line 1 and adjacent to the master jaws 5 having the jaws 2. A rod 13 is driven by a hydraulic or pneumatic cylinder (not shown) in the direction indicated by a double-pointed arrow 14 to drive all the jaws 2 in unison.
Prior to lathe machining using any of the above-mentioned chucks, a cylindrical dummy or chuck piece (not shown) having a true roundness is clamped by the leading ends of the master jaws 5 and the leading ends of the jaws 2 are machined to be located along a circle so as to prevent misalignment of a workpiece as described in detail hereinafter and to facilitate a jaw centering operation in lot production. As is well known in the art, lathe machining is carried out such that one end of a workpiece 16 is clamped by the chuck and the cylindrical outer surface and end face of the workpiece 16 are machined as indicated by imaginary lines in FIG. 4. Thereafter, the workpiece is once removed from the chuck and the machined end of the workpiece is clamped by the chuck so as to machine the remaining portion as indicated by the imaginary lines in FIG. 5. Use of the dummy as described above contributes to substantial alignment of the center of the jaws 2 with the chuck center line 1 so that centering misalignment or deviation 17 between the centers 18 and 19 of the jaws 2 caused between the first and second chucking steps can be suppressed.
However, even when the leading ends of the jaws 2 are machined for alignment as described above, the centering misalignment of each machined product is considerably greater (on the order of 3/100-5/100 mm) than expected and there arises a problem that a satisfactory degree of squareness accuracy cannot be attained at end faces of the product. The inventor made extensive studies and experiments to find out when the workpiece 16 is clamped, a phenomenon that edges 20 of the jaws 2 remote from the chuck body are moved away from the outer surface of the workpiece 16 as well as a phenomenon that a gap 24 is left between an end face 22 of the workpiece 16 and a front face 23 of the chuck body may be observed as shown in FIG. 6 even though the workpiece is in contact with the front surface 23 of the chuck body prior to the clamping steps. A possible explanation for these phenomena is that when the scroll 7 presses strongly the racks 6 on the master jaws 5 in the directions indicated by arrows 25, each rack 6 adjacent to the chuck center line 1 is displaced in the direction indicated by arrows 26 so that the whole jaws 2 are forced to tilt backwardly. As a result, accuracy of each machined product (centering alignment, parallelism, flatness and squareness) is degraded and surface condition of a machined product is also degraded by chatter vibrations. The same phenomena are also observed in the conventional hydrualic or pneumatic chucks.
In order to solve the above-described problems, there has been devised and demonstrated a drawing-in chuck which is adapted to draw and clamp a workpiece. It, however, has a problem that a clamping diameter is limited only to one size and even slight change of clamping diameter causes the clamping force and drawing-in conditions to vary over a wide range so that it may not be satisfactorily used in practice. This means that workpieces with slightly different diameters require different drawing-in chucks.
In view of the above, a primary object of the present invention is to provide a clamping device capable of drawing and uniformly clamping a workpiece along its jaw length despite of variation in diameter of the workpiece and without a gap between the end face of the clamped workpiece and front surface of a chuck.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of some preferred embodiments thereof taken in conjunction with the accompanying drawings.